
A push or a pull.
◦ When one object pushes or pulls another object, the first object is exerting a force on the second object.
◦ Like velocity and acceleration, force is also described by the strength and direction in which it acts.
◦ The strength of a force is measured in an SI unit called Newton (N). (named after Isaac
Newton)


In any situation the overall force acting on an object is the net force.

The net force determines whether an object moves and also in which direction it moves.
◦ Force in the same direction add forces.
◦ Force in opposite direction is like adding positive and negative numbers. They cancel each other out.
◦ Size of the arrows indicates the strength and direction of the force.


Unbalanced force acting on the object results in a net force which will change the objects motion.

Balanced forces equal forces acting on one object in opposite directions.
Balanced forces acting on an object will
NOT change the objects motion.


The force that two surfaces exert on each other when they rub against each other.
◦ Acts in the direction opposite to the objects motion. Friction opposes motion.
◦ Eventually friction will cause an object to come to a stop.
◦ Without friction the object would continue to move at a constant speed until acted upon by another force.


The amount of friction depends on two factors
1.
Surface Type
 Rougher surfaces have greater friction
2.
The amount of force pushing surfaces together.
 Less massive objects exert less force.

Changing the amount of the surface
(surface area) that touches does not change the amount of friction.
Determining the Amount of Friction





Salt, sand or gravel on icy surfaces
Tires on a car
Boots or shoes with traction
Other examples?


Static friction-the friction that acts on objects that are not moving
◦ Ex: Moving a desk. It takes more force to get it moving
(overcome static friction)

Sliding friction-when solid surfaces slide over each other
◦ Ex: Sliding a box across the floor, the breaks on your bike tire.

Rolling friction-when an object rolls over a surface
◦ Ex: Skateboards, bikes. Anything with wheels!

Fluid friction-when an object moves through a fluid
◦ Ex: Sailing, surfing, swimming.


Reduce
Lubricants
◦ Ex: motor oil, wax, grease.

Switching to rolling
◦ Ball bearings.

Smoother surface
◦ Plastic as opposed to wood.

Increase
Make surfaces rougher.
◦ Sand on icy roads, batting gloves.

Increase amount of force pushing surfaces together.
◦ Add weights.
Ways to Reduce/ Increase Friction


Physics of The Olympics
◦ Competition Suits
◦ Curling


The force that pulls objects toward each other.


“All objects in the universe attract each other through gravitational force . The size of the force depends on the masses of the objects and the distance between them.”
◦ Sir Isaac Newton (1642-1727)
The Law of Universal Gravitation


Mass- The more mass an object has the greater it’s gravitational pull.
◦ Ex: The sun has a large mass which keeps the planets in orbit

Distance- The farther apart two objects are, the lower the gravitational force between them.


When the only force acting on a falling object is gravity, the object is said to be in free fall.

The acceleration of any object due to gravity is 9.8 m/s/s
(ignoring air resistance).


Mass is the amount of matter in an object.
◦ Measured in grams or kilograms

Weight is the measure of the gravitational
force exerted on an object.
◦ Measured in Newtons

Why do we weigh less on the moon?
◦ Because the moon has less mass than the earth.


Weight = mass X acceleration due to gravity
(9.8 m/s/s)

How much does a 50 kg person weigh on earth?
◦ Weight = mass X 9.8 m/s/s
◦ Weight = 50 kg X 9.8 m/s/s
◦ Weight = 490 Newtons
◦ Where force (weight) is measured in Newtons, acceleration is measured in m/s/s and mass is measured in kg.

1.
2.
Convert lbs. to kg
(Weight in pounds X 0.4536)
Convert kg to Newtons
(Weight in kg X 9.8)
Ex. 190 lbs X 0.4536 = 86.18 kg
86.18 kg X 9.8 = 844.60 N
To get weight on moon just divide by 6.
How many Newtons do you weigh?


An object that is thrown is called a projectile.

Which Will Hit The Ground First?


A type of fluid friction.
◦ The greater the surface area the greater the air resistance.
◦ The greater the velocity the greater the air resistance.


As velocity increases air resistance increases. When the force of the air resistance equals the force of the gravity on an object, the object will no longer accelerate. It will continue to fall at a constant rate.

So, terminal velocity is the point at which a falling object stops accelerating and begins to fall at a constant speed.


Exploring Gravity